Japanese Patent Application Publication No. 2000-340385 describes a ballast for a discharge lamp (e.g., a high intensity discharge (HID) lamp). The ballast (hereinafter referred to as a “first prior art”) includes a DC-DC converter circuit, an inverter circuit, a start circuit and an output control circuit, and is especially characterized by the DC-DC converter circuit and the output control circuit.
The DC-DC converter circuit is a flyback converter configured to convert voltage from a DC (direct current) power supply into specified voltage, and has a transformer with primary and secondary windings, a switching device, a diode and a capacitor. The primary winding has first and second ends, and the first end of the primary winding is connected with a positive terminal of the DC power supply. The switching device is connected between the second end of the primary winding and a negative terminal of the DC power supply. The secondary winding has first and second ends, and the first end of the secondary winding is connected with an anode of the diode. The capacitor has first and second ends that are connected with a cathode of the diode and the second end of the secondary winding, respectively. The polarity of the second end of the secondary winding is the same as that of the first end of the primary winding.
The output control circuit is configured to turn the switching device on and off by either a critical mode or a continuous mode.
In the critical mode, the switching device is turned on when a secondary current through the secondary winding side reaches zero, and is turned off when a primary current through the primary winding side reaches a command value for a primary peak current. The command value for the primary peak current is made from a current command value and an output current of the DC-DC converter circuit. The current command value is evaluated from the output voltage of the DC-DC converter circuit, namely the voltage across the capacitor.
In the continuous mode, even if the secondary current is larger than zero, the switching device is turned on when an off-period of the switching device exceeds a maximum off-period. In high intensity discharges, if lamp temperature is low, an inclination of a secondary current (with respect to a temporal axis) becomes gentle by decrease in lamp voltage, and an off-period until the secondary current reaches zero becomes long. Accordingly, a switching frequency of a switching device is reduced and a peak value of a primary current for obtaining a specified output is increased, which causes increase in peak current of the switching device, a large scaled transformer and a large scaled capacitor. Especially, the influence is increased when high intensity discharges are used for a vehicle, because if lamp temperature is low, it is necessary to provide the lamp with excess electric power in comparison with a steady state in order to quickly increase a light output. By the continuous mode, the switching frequency can be prevented from be excessively decreased.
The zero point of a secondary current can be detected directly and indirectly. However, a primary current needs to be compared with the command value for the primary peak current, and accordingly requires to be detected by a signal corresponding to an actual primary current.
A primary current is detected through a resistor in general. For example, in Japanese Patent Application Publication No. H8-182314, an electric current through a primary winding is detected through a current sensor that is a resistor. However, if the resistor is used as the current sensor, power loss occurs. In highly-loaded electric power and low input voltage in particular, an electric current through a switching device is increased, and accordingly a resistor of large size must be used as the current sensor. If a low resistor is used as the current sensor, power loss can be reduced, but a sensor signal is reduced and is easily affected by disturbance (noise).
A discharge lamp ballast (hereinafter referred to as a “second prior art”) described in Japanese Patent Application Publication No. 2004-87339 can solve the problem of the current sensor. The second prior art includes a DC-DC converter circuit configured like the first prior art, and is configured to turn an and off a switching device of the DC-DC converter circuit based on a signal (voltage) value from a sawtooth oscillator and a command value. Specifically, the command value is a PWM command value, and produced in response to output voltage and output current of the DC-DC converter circuit. The switching device of the DC-DC converter circuit is turned on if the signal value from the sawtooth oscillator is smaller than the command value, and is otherwise turned off. In short, the second prior art does not need a current sensor for detecting an electric current through a primary side of the DC-DC converter circuit. In addition, the output of the sawtooth oscillator can be raised, and the influence of disturbance can be further reduced.
However, in a continuous mode of the second prior art, if input voltage or load voltage fluctuates slightly or the off timing of the switching device is slightly shifted by noise or the like, the output power can be fluctuated greatly. Specifically, when the switching device is turned off, if the output power increases slightly, an inclination of an electric current through the secondary winding becomes large. When the switching device is then turned on, an initial value of an electric current through the primary winding is reduced. An on-period of the switching device is not immediately changed by delay in output detection, delay in feedback control and so on. Therefore, when the switching device is turned off, a peak value of an electric current through the primary winding is reduced. The output power of the DC-DC converter circuit is proportional to the square of a peak current through the primary winding, and accordingly slightly shifted peak current has a great influence on the output power. In addition, in a discharge lamp having a negative resistance characteristic, the lamp voltage is increased in response to the decrease of the output. Thereby, the output voltage of the DC-DC converter circuit is further increased, and the output is decreased. Similarly, increase in output causes great fluctuation.
On the other hand, in the first prior art, even if input and output of the DC-DC converter circuit are changed suddenly, the command value for the primary peak current is unchanged. For this reason, the first prior art has few influence on output power, and even if the response of feedback control is slow, it fits in a small output fluctuation range. Therefore, the first prior art has a high output power stability in comparison with the second prior art.
Since the second prior art has no current sensor, the output power requires to be adjusted based on output voltage and output current of the DC-DC converter circuit. Therefore, if the feedback control is slow, output fluctuation is increased. For this reason, if the gain of the feedback control is increased, the response speed can be increased, but the feedback control stability becomes impaired. In order to avoid increase in allowable current of the switching device and saturation of the transformer, an overcurrent protective device is necessary in place of a current sensor.